Triplet repeat instability correlates with dinucleotide instability in primary breast cancer.

The expansion of triplet repeat microsatellite sequences is the molecular correlate of anticipation in a number of rare Mendelian neurodegenerative disorders. This finding prompted us to study these sequences in primary breast cancer in which there is evidence of genetic anticipation. We used a PCR/silver stain method to determine whether triplet-repeat instability (TRI) was present in DNA from malignant breast tumors, and analyzed microsatellite instability (MSI) in triplets SCA1, SCA2, SCA3, SCA6, HD, DRPLA and X25-GAA. We studied 54 consecutive primary breast cancers previously analyzed for dinucleotide instability (DI) at 9 loci. Microsatellite instability (TRI and/or DI) was found in 28/54 (52%) cases, ranging from 0 to 56% in each patient. Dinucleotide instability occurred at > or =2 loci in 19/54 (35%) cases and TRI in 6/54 (11%). Considering single locus instability, we found DI in 26/54 (48%) tumors and TRI in 13/54 (24%). Triplets DRPLA and X25-GAA were most frequently unstable (14% of cases); SCA2 instability was not detected. Interestingly, most tumors with TRI had DI (11/13, 85%). There was a correlation between TRI and DI in the same tumor (42 vs 7% in DI+ and DI- tumors respectively, p=0.0028). Furthermore, TRI appears more frequently associated with lymph node metastases and more advanced clinical stages and more frequent in patients <50 years old, with positive steroidal hormone receptor status, positive p185 and negative p53. These findings are of interest because they demonstrate a relationship between TRI and the clinicopathological characteristics of cancer aggressiveness. Triplet repeat alterations can interfere with gene expression and proteomic functions, which suggests they can play a role in the neoplastic progression of mammary cells. Furthermore, the association of TRI and DI in the same tumor suggests that alterations in the DNA repair gene could culminate in selective phenotypes and breast cancer progression in a considerable number of patients.

Loss of hMSH2 expression in primary breast cancer with p53 alterations.

Inactivation of DNA mismatch repair genes (MRG) is a recently described pathway of cancer development and progression resulting in genetic instability. Germline mutations in MRG have been studied predominantly in patients with hereditary non-polyposis colorectal cancer (HNPCC) where it is associated with microsatellite instability (MSI). The expression of MRG in primary breast cancer is still largely unexplored. The hMSH2 MRG encodes a protein that recognizes and binds to mismatch sequences of DNA. We investigated the relation-ship between hMSH2 expression and clinicopathological and biological characteristics, including p53 and p185 expression, in 44 primary invasive breast cancers. hMSH2 was not expressed in 11 cases (25%). Interestingly, p53 (p=0.05), p185 and steroidal receptor expression (p=0.07) were more frequent in tumors without hMSH2 expression. Furthermore, in 30 of 44 cases we analyzed hMSH2 expression in relation to MSI at 9 dinucleotide loci, and found that MRG expression was not significantly related to MSI. The presence of hMSH2 and p53 alterations in the same tumor suggests that the two oncoproteins act through a common mutational pathway, whereas the absence of a correlation between hMSH2 and MSI suggests that oncogenetic mechanisms of progression in primary breast cancer differ from those in HNPCC.

p53 expression is decreased in primary breast carcinomas with microsatellite instability.

p53 and p185 expression in primary breast cancer with microsatellite instability (MSI) is still largely unexplored. To investigate the relationship between these oncoproteins and the pathways of genomic instability, we examined 52 primary invasive breast cancers stratified by the presence and absence of MSI. We determined the status of eight microsatellite loci using radioactive and silver staining methods, and evaluated the immunohistochemical expression of p53 and p185 in a consecutive series of Italian cancer patients characterized by clinical-pathological and biological parameters. Nineteen cases (36.5%) were MSI-positive in at least two loci. p53 was expressed in 15 cases (28.8%) and p185 in eight (15.4%). MSI-positive tumors were inversely correlated with p53 expression (p = 0.0007); in addition, the percent of p53-expressing cells decreased as the number of MSI-positive loci increased. MSI-positive tumors were correlated with a larger tumor size (p = 0.04), lymph-node metastasis (p = 0.001), and advanced clinical stage (p = 0.0006). These data demonstrate the existence of two subsets of primary breast cancers: one characterized by MSI, the other by p53 expression. MSI-positive patients had a more advanced and/or aggressive disease.